Mechanical Principles: Springs, Bearings, and Material Forces

Mechanical Principles

Static and Dynamic Forces

• Static Force (Stationary):

  • Forces caused by the weight of the structure and anything permanently attached.

  • The force is stationary.

• Dynamic Force:

  • Forces that are moving and caused by people, mechanisms, or the natural environment.

  • Dynamic forces are usually much greater than static forces.

  • Difficult to predict; experience helps in designing, planning, and production stages.

Tension

• Tension is the force that tends to pull things apart.

• It is the pulling force that attempts to stretch or lengthen.

• $F_L$ represents the tension force. (driven)

Compression

• Compression is the force that tends to push materials together.

• It is the pushing force that tries to squash or shorten.

• $F_L$ represents the compression force. (driven)

Torsion

• Torsion is the force that causes a twisting motion.

• A torsional force tends to distort by twisting.

• Example: Twisting a plastic ruler between both hands.

• $F_L$ represents the torsion force. (driven)

Bending

• Bending is a combination of tension and compression.

• Bending is the force that attempts to cause bending deformation.

• $F_L$ represents the bending force. (driven)

Bending Forces Explained

• When a beam supported at each end is loaded:

  • The top is in compression.

  • The bottom is in tension.

• Neutral Axis is the point where there is neither compression nor tension.

Shear

• Shear is the force that tends to slip or slide one part of a material with respect to another part.

• Examples:

  • The surface between the handle and the body of a cup.

  • A pair of scissors: the two handles put force in different directions on the pin that holds the two parts together. The force applied to the pin is called shear force.

• $F_L$ represents the shear forces.

Springs

• A spring is a mechanical device used to store energy and subsequently release it when it is required.

• Functions:

  • Absorb shock (e.g., suspension on vehicles).

  • Maintain a force between contacting surfaces (e.g., clips and pegs).

• Made of an elastic material, generally a steel with alloys added to improve flexibility, durability, and elasticity.

Helical Springs (Coil Springs)

• Coil springs are made of an elastic material formed into a helix shape (usually by winding wire around a cylinder).

• Three common types:

  • Compression:

    • Designed to resist compression.

    • Free length will have a pitch.

    • Placing load on the spring will reduce the free length.

  • Extension:

    • Designed to resist stretching.

    • Placing load on the spring will increase the free length.

  • Torsion:

    • Designed to resist rotation around an axle.

    • The helical torsion spring experiences bending stress, not torsional stress as the name suggests.

Leaf Springs

• Leaf springs are useful to accommodate large loads (like a trailer or truck) with a relatively low displacement.

• They can be adjusted by adding or subtracting ‘leaves’ to increase or decrease the load that they can bear.

• They accommodate the load as a beam would bend.

Bearings

• The function of a bearing is to minimize the friction between moving parts and to carry a load.

• Types:

  • Simple plain bearing or sleeve.

  • More complex roller-type bearing.

• Most bearings in machinery such as gearboxes and engines are metal roller bearings.

Ball and Roller Bearings

• Consist of:

  • An inner ring.

  • An outer ring.

  • A number of rolling elements (balls or rollers).

  • A cage.

• Most bearings are made of steel alloyed with chrome to be hard-wearing with a very high-quality surface finish.

• Some very high-temperature applications can utilize ceramic bearings.

Types of Bearings

• Ball Bearing:

  • Have very low friction due to the contact between the cage and inner/outer race.

  • Can bear a moderate load.

  • Narrow on axle with a large cross-sectional area.

• Needle or Roller Bearing:

  • Higher friction than ball roller bearing.

  • Can bear a high load.

  • Thin cylindrical roller.

  • Low cross-sectional area.

Thrust Bearing

• Also support load of rotating parts, but in this case, the load is along the axis (axial load).

• Found in swivel chairs.

• Rollers (cylindrical) are used for higher load applications, and balls are used for medium loads.

• Tapered rollers can be used to provide support for loads that include both axial and radial.

• Often seen in automotive applications such as wheel stubs.

Linear Bearing

• Can also be used to reduce friction between components that are moving in a linear or reciprocating motion.

• Often used to facilitate the motion of beds and machine tool heads in CNC machines, mills, and routers.